DataAnnotations, MVC and LINQ To Entities

First of all I would like to welcome any first time visitors to my blog. Before I start this blog I would like to give a brief introduction to myself. My name is Mark Anthony Farrugia and, apart from being a Computing Teacher, I am a freelance web developer based in Malta, a small island in the Mediterranean. So now let me start this blog post :) .

In this blog I will be covering some useful features called DataAnnotations which one can use when using LINQ To Entities, together with MVC. The outcomes of this blog are

  1. Understand and learn how you can create DataAnnotations on your LINQ To Entities generated model
  2. Understand how these annotations link with MVC Views which are then used to modify database data
  3. Understand how validators are then utilized automatically by ASP.NET following the annotations you create

The prerequisites of this blog are

  1. Have some understanding of LINQ To Entities
  2. Have some understanding of MVC and how you can create records using ASP.NET MVC
  3. Have SQL Server Express installed and have a database containing a Products table. You can install SQL Express from here.

Whenever you create an ASP.NET MVC Template Project using Visual Studio an AccountModel inside the Models folder will be automatically generated. If you open this class you will note that there are some very useful properties which ASP.NET MVC uses to validate data inputted by the user. Let’s take a simple example.

public class LoginModel
 {
 [Required]
 [Display(Name = "User name")]
 public string UserName { get; set; }

[Required]
 [DataType(DataType.Password)]
 [Display(Name = "Password")]
 public string Password { get; set; }

[Display(Name = "Remember me?")]
 public bool RememberMe { get; set; }
 }

If you examine the above code you will note that on some specific fields there are what so called DataAnnotations. These DataAnnotations are

  1. Display (line 4,9 and 12)
  2. DataType (line 8)
  3. Required (line 3 and 7)

What are these DataAnnotations and why are they used? These DataAnnotations basically give MVC some extra information on what you expect from the user when inputting data in an MVC View. For example, the Required DataAnnotation basically ensures that the specific field cannot be left empty. That would mean that a user would not be allowed to leave the field empty when inputting data. Thus, in the example shown above, the Username and Password fields are both required.

The Display DataAnnotation, on the other hand, is used so that the label associated with the field, when displayed in an ASP.NET MVC View, will display the relevant Display information.

The DataType DataAnnotation will provide information on the DataType of that specific field. So for example, the Password field is in our case a Password field. This means that whenever you will be creating a View which accepts a LoginModel class, the Password field will be displayed as an HTML Input Type = password field.

The example which the ASP.NET MVC template project is very useful to let use understand how DataAnnotations can be used. However, what happens when you are using LINQ To Entities and you would have your classes already specified in your model. How can you use these very useful DataAnnotations and apply them to your LINQ To Entities model?

Basically my main aim is not to create DataAnnotations and apply them on a LINQ To Entities model which we have created in this blog. Thus, from now on I will assume that you have a LINQ To Entities class named Product which basically stores products information.

The first thing you will have to do is to create a partial class with the same name as your LINQ To Entities class. So ,in our example we will name our partial class Product. We would then create another class which will contain the MetaData containing all the fields in our LINQ To Entities Product model which we would like to apply DataAnnotations on. This code is shown underneath.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel.DataAnnotations;
namespace MVCForBlog.Models
{
[MetadataType(typeof(ProductMetaData))]
 public partial class Product
 {
 }
public class ProductMetaData
 {
 [Required(ErrorMessage = "Product Name is required")]
 [Display(Name="Product Name")]
 [DataType( System.ComponentModel.DataAnnotations.DataType.Text)]
 public string ProductName { get; set; }

[Required(ErrorMessage = "Product Description is required")]
 [DataType( System.ComponentModel.DataAnnotations.DataType.MultilineText)]
 [Display(Name = "Product Description")]
 public string ProductDescription { get; set; }
}
}

Before I go into the detail of what the above code does I will have to make some points very clear

  1. To use DataAnnotations you should Add A Reference to the System.ComponentModel if this has not already been added
  2. You should ensure that the partial class is in the SAME namespace as your LINQ To Entities model. THIS IS CRITICAL TO MAKE ANNOTATIONS WORK AS EXPECTED.
  3. The fields you create in your class should have the SAME name as the field name specified in the LINQ To Entities model.

If you investigate the code listed you will note the following

  1. From line 8 to line 11 we are creating a partial class with the same name as the class found in our LINQ To Entities model. We are then specifying that it will be using the MetaData created in a separate class named ProductMetaData
  2. The ProductName field specified in line 17 has several data annotations applied. These include Required, which means that the user would not be able to leave the field empty, DisplayName which means that the field which will be displayed near the textbox will be shown as Product Name, and finally DataType which basically specifies what type of data the user should be inputting. In our case the ProductName is a string and thus we are specifying Text as the DataType.
  3. The ProductDescription found in line 22, has the same DataAnnotations as in the previous field. However the DataType for this field is MultiLine text. Thus when this field will be displayed in an ASP.NET MVC View a TextArea will be shown.

Now let’s see how these annotations will affect the ASP.NET MVC View which will be used to create a new product to our database.

Create an ASP.NET MVC View which will create a new product to the database as shown in Figure 1. You will note that when you run your application and try to create a new product, the fields and GUI elements which will be created will reflect the DataAnnotations specified.

Figure 1 - ASP.NET MVC View using Annotations

Figure 1 – ASP.NET MVC View using Annotations

So basically if you look at the above screenshot you will note that the ProductName and ProductDescription fields have a label as specified in the Display DataAnnotation found in our metadata class. You will also note that the ProductDescription field is an HTML TextArea since the DataType Annotation for this field is set to MultiLine.

You will also note that if you try to create a new product and leave the required fields empty, ASP.NET MVC will display a validation error, both on the ProductDescription and on the ProductName since both of them have a Required DataAnnotation. You will also note that the error displayed is the same error as specified in the Required ErrorMessage property. The validation errors are shown in Figure 2.

Figure 2 - MVC View with Validation errors

Figure 2 – MVC View with Validation errors

I hope you found this blog post useful. And till next time… take care of yourselves :)